1. Field of the Invention
The present invention relates generally to apparatus for marking a well casing depth, and more particularly to a magnetic device adapted to actuate a casing collar locater log.
2. Description of the Prior Art
At the completion of drilling an oil or gas well, hydrocarbon bearing formations are usually detected and located by means of logging tools run on the end of electrical cables inside the drilled or open hole. If it is determined from these open hole logging devices that production from the hydrocarbon formations is feasible, then casing is run across these formations and the annular area between casing and hole is filled with cement.
In order to accurately perforate the producing formations through casing and cement so that hydrocarbons can flow into the well, it is essential the locations of these producing formations be established so that the perforating gun can be properly positioned inside the casing prior to its firing. This is typically performed with cased hole logging tools, which usually include a radioactive tool run in tandem with a casing collar locating tool.
The radioactive tool is able to look through the casing and detect and locate the same producing formations logged initially in the open hole and the collar locating tool defines where the casing joints are coupled together with respect to the producing formations. The location of these formations and the casing collars are recorded simultaneously to produce a correlation log.
Although it would seem that the perforating gun could be positioned inside the casing simply by honoring depth measurements taken by directly measuring the electrical cable spooled into and out of the hole, large discrepancies would occur in the actual downhole position of the perforating gun with respect to depth measurements taken with either the open hole or cased hole correlation log. This would occur because of differences in:
1. Cable and tool weights and their physical properties; PA0 2. Downhole conditions (temperature, pressure, directional deviations); PA0 3. Surface elevations which alter cable tension which results in elongational differences. This cable stretch or slack will in turn cause subsequent depth correlation errors.
It also would seem simple to position the perforating gun by running it in conjunction with the radioactive tool used in obtaining the correlation log. However, since these radioactive tools are extremely delicate and expensive and since tremendous shocks are generated when perforation charges are fired, perforating guns are normally positioned utilizing a collar locater in tandem with the perforating gun.
Although the collar log method of positioning perforating guns has been reduced to common operating practice, it is not a foolproof positioning means.
For example, most casing collars are approximately equidistant apart along the casing and it is difficult to keep track of precisely which collar the collar locater has located. This problem is enhanced in very deep wells where there are naturally more collars to keep track of. This problem becomes more severe when reperforating an older well, because it is not uncommon to have depth variations as much as 20 feet.
In these instances the operator is faced with guessing which collar located with the collar log corresponds with its counterpart on the correlation log. This guess would be in lieu of making some positive collar correlation that would define the position of the perforating gun in the hole.
Another common example is a failure of the casing collar locater to detect the casing couplings. This occurs when premium-seal or integral joint pipe couplings are employed, when non-magnetic alloy tubulars for corrosive environments are used, or when casing that has been magnetized by drilling operations that have been conducted through said casing are logged. In these cases, the operator is forced to rerun a correlation log and work from depth measurements, which has its inaccuracies as previously described, or tie into some reference point a considerable distance away from the formation, such as the bottom of the well, which itself is only an approximation. In the above instances, the operator is forced into making an erroneous attempt to perforate the well.
There have been numerous attempts to positively and unambiguously mark a location, or reference point upon a casing; however, none of the prior devices have been entirely successful. One such device comprises a collar adapted for attachment about the exterior of the casing. The theory of operation of that device is that the casing collar locater log is responsive to metal mass in its vicinity. However, that device has not proved successful and it has been determined that the casing collar locater log is responsive to changes in internal diameter of the casing rather than metal mass.
Another device, shown in U.S. Pat. No. 3,106,960, comprises a collar adapted for placement about the casing, and includes a pair of diametrically opposed magnetic inserts on the interior of the collar. The magnetic field produced by the device of the '960 patent has little or no tendency to penetrate the casing, and therefore the device is virtually undetectible.
Other devices have been proposed, which place magnets on the interior of the casing. Examples of such devices are shown in U.S. Pat. Nos. 3,513,912; 3,105,551; 3,105,546; 3,105,548; 3,105,549; 3,105,550; 3,171,486; 3,171,488; 3,187,815; 3,198,255; and 3,105,547. All of those devices are disadvantageous in that they must be used with specially fabricated sections of casing and are therefore not usable with standard commercially available casing.